Polystyrene tear back lids are commonly used in foodservice applications, in particular as covers for hot cups. For example, U.S. Pat. Nos. 4,460,103, 5,490,609 and 5,699,927 (the disclosures of which are incorporated herein in their entireties by this reference) disclose various types of tear back lids. It is also known that the tearability of a polystyrene tear back lid can be improved by designing the lid such that the tear back portion is oriented in the direction of extrusion. U.S. Pat. No. 5,613,619 (the disclosure of which is incorporated herein in its entirety by this reference) discloses such a feature.
Recently, manufacturers of polystyrene lids for hot cups have begun to investigate inclusion of filler into the lids to reduce the costs of manufacturing such lids. Historically, polystyrene lids for hot applications have not included a significant amount of filler. This is due primarily to two reasons. First, polystyrene has traditionally been a low cost raw material and, as such, there was little motivation to include filler into a formulation. Further, polystyrene used in hot cup lid applications is generally high impact polystyrene (“HIPS”). HIPS is FDA compliant and exhibits good thermoformability due to its low brittleness. Since filler is known to increase the brittleness of polystyrene, it was not desired to negate the low brittleness of HIPS with the addition of filler, since this was a property for which HIPS was selected for use in thermoformed hot cup lid applications.
The inventors herein have surprisingly found that filler can be added within a specified range to provide a suitably tearable filled thermoformed HIPS container lid when the tearback portion of the lid is oriented in the extrusion direction of the polystyrene when the tear back lid comprises two sets of tear indentations and a tab portion.